The ultimate objectives of this program are to acquire information on the nature of the molecules that turn on and turn off transcriptional processes in eucaryotes and to search for the mechanisms by which these processes are integrated into programs of development. In our specific approach to these problems, we have made use of Drosophila melanogaster as the experimental organism. Four highly specific and unique phenocopies involving structures of bristles and hairs have been produced by heat shocks during a twelve-hour pupal period and corresponding mutants have been studied. We have demonstrated that during the sensitive periods for phenocopy induction the hypodermal cells which are involved undergo rapid and extreme changes in patterns of proteins that are being synthesized. We have also demonstrated that the heat shock applied shuts down both protein synthesis and RNA synthesis. These synthetic capacities return after a few hours but the recovery is neither complete nor normal in the sequence of events. It appears that a common ground for mutant and phenocopy expression can be found in these processes and that the system can be used advantageously for studies of developmental regulation.